Final answer:
The numeric constant of variation in this relation is 100.
Step-by-step explanation:
To find the numeric constant of variation, we can set up a proportion using the given information. Since y varies directly as x, we can write the equation y = kx, where k is the numeric constant of variation.
Using the given values, we have 400 = kr and r = k(4).
Solving these equations, we find that k = 100. Therefore, the numeric constant of variation in this relation is 100.